1. Field of the Invention
The present invention relates to an electromagnetic valve having a valve member slidable in an axial direction thereof and capable of causing a reciprocative movement in response to activation and deactivation of an associated solenoid generating magnetic field.
2. Prior Art
FIG. 5 shows one example of conventional electromagnetic valves which basically accommodate a plunger type magnetic circuit therein, chiefly used as an electromagnetic valve adjusting an overflow or returning fuel amount of a fuel injection pump for an internal combustion engine. Unexamined Japanese Patent Application No. SHO 61-72867, corresponding to the U.S. Pat. No. 4,653,455, shows another type electromagnetic valve accommodating a plane type magnetic circuit.
In FIG. 5, an electromagnetic valve 100 comprises a casing 101 with one end having a cylindrical bore therein. A valve body 102, communicated with a high-pressure pump chamber (not shown), is fixedly housed in the cylindrical bore of the casing 101. A stationary cylinder 103 has a flange 103a fixed to the cylindrical bore of the casing 101. A movable cylinder 104 is slidably supported on the inside wall of the valve body 102. A valve needle 105 is slidably supported on the inside wall of the stationary cylinder 103, and a compression coil spring 106 gives the valve needle 105 a biasing force so as to open the valve. A shim 107 of disk-like shape is interposed between the casing 101 and the stationary cylinder 103 along the axis of the electromagnetic valve 100. The shim 107 regulates the axial movement of the valve needle 105. The casing 101 has the other end having a smaller-diameter bore 101a and a larger-diameter bore 101b therein. The smaller-diameter bore 101a fixedly accommodates a cylindrical bush 110 along its inner wall. The cylindrical bush 110 allows a push rod 111 to slide along the inside wall thereof. The push rod 111 is brought into contact with the valve needle 105 at one end thereof. The other end of the push rod 111 is connected with the armature 112. The armature 112 is slidable along the inside wall of a stator 108, and a compression coil spring 113 gives the armature 112 a biasing force so as to open the valve. A coil 115 wound around a bobbin 114 is housed between the smaller-diameter bore 101a and the larger-diameter bore 101b. Both ends of the coil 115 are connected to a terminal 116. The terminal 116, entering into a through hole 108a formed on the stator 108, supplies electric current from a control circuit (not shown) to the coil 115. The biasing force of the compression coil spring 106 is larger than that of the compression coil spring 113. When the coil 115 is not supplied with electric current, the biasing force of the compression coil spring 106 shifts the valve needle 105 to open the valve.
Until fuel force feeding is finished, the coil 115 receives electric current and generates an electromagnetic attraction force, with which the valve needle 105 is urged to close the valve against the biasing force of the compression coil spring 106. When the fuel force feeding is finished, electric current to the coil 115 is stopped and the biasing force of the compression coil spring 106 acts to return the valve needle 105, thereby opening the valve. Thus, fuel overflows or returns from the high-pressure pump chamber to a low-pressure fuel chamber through an overflow hole 102a of the valve body 102 and an overflow or return passage 101c formed in the casing 101.
On the other hand, the electromagnetic valve disclosed in the Unexamined Japanese Patent Application No. SHO 61-72867, corresponding to the U.S. Pat. No. 4,653,455, adopts an arrangement in which a coil is extracted outward further than the outermost periphery of an armature and connected to a terminal, thereby preventing the armature from interfering with the terminal when the armature extends radially outward.
However, the electromagnetic valve shown in FIG. 5 is disadvantageous in that the armature 112 is housed radially inside the terminal 116. It means that an area of magnetic flux generated from the coil 115 and passing through the armature 112 is very small. Thus, a magnetic attraction force acting on the armature 112 is not satisfactory. This results in a problem of poor response in the fuel overflow or return characteristics. To improve such poor response, it is normally required to increase electric current to be supplied to the coil 115. However, such an increase of electric current will directly increase electric power consumption.
Moreover, the electromagnetic valve disclosed in the Unexamined Japanese Patent Application No. SHO 61-72867, corresponding to the U.S. Pat. No. 4,653,455, is disadvantageous in that clearance adjustment between the stator and the armature is required at two places, resulting in increase of production cost.